1. Field of the Invention
The present invention relates to a dual sensor intrusion detection system which utilizes adaptive sensor detection techniques to reduce false alarms.
2. Description of the Related Art
Typically, dual sensor intrusion detection systems include a passive infrared radiation (PIR) detector and a microwave detector, both of which are designed to protect the same volume of space. Most known dual sensor intrusion detection systems continuously operate the PIR detector and the microwave detector in the active mode, i.e., both are capable of detecting an intruder simultaneously. However, in operation, these dual sensing systems have a small false alarm rate which may be improved upon. Typically, this small false alarm rate is the result of two factors either in combination or standing alone. The first factor being a lack of signal processing networks and the second factor being an overall high sensitivity in current dual sensing systems which cannot be modified to respond to disturbances and/or other environmental conditions.
An alternate type of dual sensor intrusion system is one which turns one of the sensors off until the active sensor detects an intruder. As an example, U.S. Pat. No. 4,882,567 to Johnson inactivates the microwave sensor until the PIR sensor detects an intruder. However, this configuration still renders dual sensor intrusion detection systems susceptible to false alarms, in addition to reducing the sensitivity for the overall system.
One approach to improving the detection capabilities of the system has been to increase the sensitivity of one of the sensors after the other has detected an intruder. For example, U.S. Pat. No. 4,437,089 to Achard discloses a dual sensitivity intrusion detection system which increases the sensitivity of a second zone of sensors after the first zone of sensors detects an intruder. However, increasing the sensitivity of the second zone of sensors increases the likelihood of detecting the intruder, but still leaves the intrusion detection systems susceptible to false alarms, especially false alarms caused by disturbances, such as falling objects, rodents, electrical transients, vibrations and/or other environmental conditions, e.g. heat sources.
Therefore, a need exists for dual sensor intrusion detection systems which can operate with only one sensor active, while the other sensor remains inactive, until the active sensor detects motion. Further the need exists for a dual sensor intrusion detection system which reduces false alarms caused by disturbances and/or other environmental conditions within the protected area and which maintains the stability of the system.